1. Field of the Invention
Embodiments of the inventive concept relate to a signal providing apparatus, and an analog-to-digital converting apparatus and an image sensor using the same.
2. Description of the Related Art
An image sensor is commonly used in applications such as digital photographs, scanners, machine vision systems, monitoring cameras, and the like. While a charge coupled device (CCD) image sensor has been used in these applications for a relatively long period of time, a complementary metal oxide semiconductor (CMOS) image sensor is increasingly being used instead the CCD image sensor. CMOS image sensors are advantageous in that, for example, like an image processing circuit, the CMOS image sensor can be fabricated together with peripheral circuits on the same chip, and thus, CMOS image sensors can be configured as a “camera on a chip” and can be used in other applications. Also, since the CMOS image sensor is fabricated by using a general CMOS fabrication technique, CMOS image sensor fabrication incurs low fabrication costs in comparison with fabricating CCD image sensors.
FIG. 1 is a view showing a configuration of a related art image sensor.
An image sensor generally uses a “column parallel architecture” in which a plurality of image pixels are arranged to have a square shape with rows and columns. Image pixels in respective columns are connected to column data lines, and the respective column data lines are selected row by row so as to be driven.
With reference to FIG. 1(a), the prior art image sensor—comprising photo diodes, amplifiers, select switches, column lines, and correlated double sample modules, among others—receives analog image signals output from all the image pixels of a column and converts the analog image signals into digital signals by using a single analog-to-digital converter (ADC).
According to the ADC scheme of FIG. 1(a), it can be seen that as the degree of image sensor integration is increased, the operating speed of the ADC should be linearly increased. Namely, in order to output data with respect to an image frame at the same rate, the ADC should process data from all pixels belonging to the frame at the same time (simultaneously) and the speed of the ADC should be increased in proportion to the number of pixels. However, a high speed ADC is costly and it may be difficult to implement such a high speed ADC.
Also, since the speed of outputting data in columns should be increased, more noise is inevitably generated due to data interference between columns.
With reference to FIG. 1(b), recently, the related art image sensor may employ a column-parallel-ADC scheme in which ADCs are disposed by column and an analog image signal is converted into a digital signal by column.
According to the ADC scheme illustrated in FIG. 1(b), an operation speed of the ADC may be required to be proportional to the number of rows, so it is proportional to a square root of the degree of integration. Thus, in comparison to the conventional scheme, the ADC may be designed to have a low speed. In addition, since analog signals are converted by column, column fixed pattern noise can be reduced.